The pulp and paper industry is one of the biggest industries consuming a great amount of vegetal resources (wood) and water in the course of processing these. Approximately 50 percent of the initial material, after being processed, goes to wastes, liquid and solid ones. Wastewater amount and its degree of contamination depend on the degree of elaboration of the plant flow diagram. In the pulp and paper industry, wastewater may be subdivided, by the nature of contaminations occurent therein, into lye-containing, acidic, fetid, bark-containing, fibre-containing, and sludge-containing ones. Suspended matters, such as fibre, bark and the like, can be removed mechanically from the sewage, for example in primary sumps. For purification of wastewater from organic contaminations dissolved therein, biological purification is of the highest effectiveness. The gist of the wastewater biological purification process is in consumption of the dissolved organic substances by microorganisms, part of the organic substances being oxidized, part of them being transformed into biomass.
Enabling a deep degree of purification, the wastewater biological purification method, however, entails a penalty of considerable amounts of waste activated sludge formed therewith. The yield of the waste activated sludge constitutes from 160 to 210 grams per cubic meter of the water purified. The waste activated sludge is substantially an amorphous flocculation mass seeded densely with aerobic bacteria and other microorganisms. By its mechanical composition, the activated sludge relates to fine suspensions, 98 percent by weight of which consists of particles less than 1 mm in size. Activated sludge is also characterized by a high water content. It is precisely these features that the complexity of treating the activated sludge mass is accounted for, since a great amount of energy is required to remove the tightly bound. Moreover, decantation of the sludge by filtration methods fails to provide complete dewatering and results in filter stopage, which in turn entails the necessity of periodical restoration of the filtering capability of the strainers. Disposal of the activated sludge into natural basins in that form which it takes at the purification stations is not allowable since this would result in silting of the basins and disturbance of natural processes occurring therein. Because of this, attempts have been made to develop methods for treating waste activated sludge in a way promoting its utilization.
Fibre- and bark-containing wastewater are another sources furnishing the pulp and paper production sewage with the residues. Fibre-containing wastewater is issued when manufacturing half-finished products, paper, cardboard and wood-fibre board. Removal of the fibre-containing sewage by dilution is not allowable since the fibre decomposition results in basin pollution. Bark-containing sewage is issued upon wet peeling. When ingressed in natural basins, the rind can form benthal deposits which are inimical to aquatic life. In consequence fibre- and bark-containing wastewater is to be purified and the residues consisting of fibre and bark are to be eliminated.
There is known a number of methods for recovery of wastewater residues followed by utilization of these in the manufacture of products at the pulp and paper mills. Among these methods is utilization of the slurry-lignine formed upon biological purification of wastewater as a filler, in an amount of 0.6-15 mass percent by absolutely dry basis, when manufacturing packing material (Cf. USSR Inventor's Certificate No. 503,967, Int. Cl. D21H 3/00, D21D 3/00, issued Feb. 25, 1976).
According to USSR Inventor's Certificate No. 440,468 (Int. Cl. D21H 3/00, issued Jan. 31, 1975) raw activated sludge in an amount of up to 20 mass percent by absolutely dry basis is introduced into the whole stuff for manufacturing of packing board.
Utilization of raw activated sludge in this composition of the stocks results in an increased flow rate of steam in the pressing portion of the paper machine because of the necessity of removal of the tightly bound moisture contained in the activated sludge and, consequently, utilization of the activated sludge biomass in operating conditions doesn't exceed 3 percent and constitute 13 kg per ton of the products.
According to USSR Inventor's Certificate No. 536,268 (Int. Cl. D21C 1/00, D21C 3/02, issued Mar. 4, 1977), wood chips are held, before pulping, in an activated sludge suspension. Such a treatment leads to an increased yield of the pulp and improved mechanical properties of the latter. Analogous results are obtained when using activated sludge biomass directly for puling instead of a portion or the whole amount of the black liquor (Cf. Bukhteev B. M., Ishkhanov V. A., Grudinin V. P., Dergunova T. V. "Ispolzovanie izbytochnogo aktivnogo ila v tekhnologii proizvodstva sulfatnoy tselulozy" Referativnajy informatsija "Tseluloza, bumaga i karton", 1979, No. 15, pp. 3-4).
Sulfate pulping (kraft process) is carried out in strong white liquor admixed with black liquor. Strong white liquor is prepared in the following way. After pulping, waste black liquor goes for combustion, in the course of which the organic portion of the black liquor is burnt out, and the mineral portion--sodium salts--forms the melt. The principal components of the melt are sodium carbonate (Na.sub.2 CO.sub.3) and sodium sulfide (Na.sub.2 S). The melt is then dissolved in weak white liquor resulting upon washing the white slurry, the solution of melt in weak white liquor being referred to as green liquor. The principal component of green liquor--sodium carbonate--represents a non-active reagent in the sulfate pulping of wood and, because of this, the sodium carbonate is converted to an active pulping reagent--sodium hydroxide--i.e. causticization is effected. The causticization is carried out by adding to the green liquor sufficient lime, the causticization process resulting in the formation of strong white liquor and white slurry (CaCO.sub.3). The strong white liquor, which is substantially a mixure of NaOH and Na.sub.2 S, is used in the pulping.
Thus, the substitution of the waste activated sludge for a portion or all of the black liquor fails to utilize, in kraft process, all the waste sludge collecting at the purification stations.
There is known a method for the elimination of wastewater residues by disposal of the previously dewatered and dried residue to a refuse tip. Such a method for the elimination of wastewater residues involves the erection of new residue storage facilities as the existing ones become filled, which, in turn, requires more area. Additionally, the residue storage facilities exert adverse condition in the area, from the viewpoint of the environmental protection. Because of this, methods for the elimination of wastewater residues comprising dewatering, drying and incineration of these are preferably used.
Known in the art is a method for the elimination of wastewater residues, in particular, of waste activated sludge, comprising its coagulation by iron chloride solution and lime milk followed by dewatering in vacuum filters, drying, and incineration (Cf. Jakovleva O. I., Tkatchenko N. I. "Otchistka stotchnyh vod", "Lesnaja promyshlennost" Publishers, Moscow, 1975, pp. 40-42).
The method is implemented as follows. In a sludge packer, the waste activated sludge biomass is mixed, in a ratio of 1:1 or 1:2, with the residue from the primary sumps consisting of fibre and bark. To achieve coagulation of the residue, it is treated with iron chloride and lime milk. The residue is then dewatered in vacuum filters, preferably in ones with descending plate of the filtering fabric. The dewatered residue is removed from the plate of the vacuum filter with a special knife or it is blown with compressed air. The dewatered residue having a humidity of 83-85 percent is subjected to thermal drying in drying drums at a temperature of 500.degree.-800.degree. C. The dried residue having a humidity of 20-40 percent is incinerated. The elimination of wastewater residues by said method involves the erection of thermal drying and incineration shops that consume much energy. Moreover, the filtrate decanted upon dewatering the precoagulated residue and discharged on the purification structure contains toxic compounds of iron and calcium, which influences adversely the process of biological purification of wastewater. Dewatering of the residue in the vacuum filters is accompanied by clogging of the filtering plate which impairs its filtering capability. To restore the filter plate, it must be continuously washed through the sprinkling system and periodically rinsed with inhibited muriatic acid.